Gent Master Clock

The moment of Impulse is just about to occur. The 'count' wheel arm (correctly called a driving pawl or gun lever), having swung to the right allowing its pin (circled blue) to drop into the deeper tooth, has pushed the latching arm (or stirrup) out of the way (circled red).

This allowed the impulse roller to drop down onto the pallet.
The pendulum is now swinging back again to the left with the roller on just the top face of the pallet at this stage. As the pallet at this point is just a radius of the point of rotation (the suspension spring at the top), it is not actually giving any impulse to the pendulum (correctly the pallet has what's called a 'dead face' at this point).
But very soon however, as the pendulum gets to the mid point of its swing, the roller will drop onto the sloping face of the pallet and then it will push the pendulum to the left giving it an impulse.

Impulsing a pendulum at its mid point of swing will always give the least disturbance to the pendulum.